Based on previously reported individuality of serum chemical and hematological constituents uniquely characterizing a person's state of health, a working hypothesis is being tested that the individual profile of quantitative measurements as defined by a time-series of values over a period of optimum health, constitutes a personal reference state. This persists during optimum health. A departure (deviation beyond the reference limits) of one or more characteristics or a composite index, although within the conventional "normal range" for a population may not only predict development of abnormality but indicate the desirability of initiating preventive measures to reverse the detected trend. The establishment of individual reference profiles, periodic monitoring and appropriate intervention constitute a sensitive and one effective approach to health maintenance and preventive medicine. This continuing study is designed to test this hypothesis and answer related questions including: 1. What are relevant chemical, physiological and psychological measurements? 2. What are adequate numbers and time patterns of measurements for reference profiles? 3. What are definable criteria of optimum health? 4. What are optimum intervals for monitoring an individual's changing health? 5. What are criteria for recognizing "departures"? 6. How can departures be interpreted in relation to extrinsic (environmental) factors and subsequent developments of abnormality? 7. What measures of tolerance to challenge and stress are relevant to the individual reference profile and of predictive sensitivity? These require long-term (15-20 years) studies of initially healthy people; the development of biomathematical models of computer processed data, innovative statistical analyses and automated, precise and cost-effective testing procedures.